The invention described herein relates to nuclear reactor fuel assemblies and more particularly to a reconstitutable assembly having removable top and bottom nozzles.
Conventional designs of fuel assemblies include a multiplicity of fuel rods and control rod guide thimbles held in an organized array by grids spaced along the fuel assembly length. Top and bottom nozzles on opposite ends thereof are secured to the control rod guide thimbles which extend slightly above and below the ends of the fuel rods. At the top end of the assembly, the guide thimbles are welded in openings provided in the top nozzle while those at the lower end are attached to the bottom nozzle by a shoulder bolt which is locked and then welded in position.
During operation of such assembly in a nuclear reactor, the fuel rods may occasionally develop cracks along their length resulting primarily from internal stresses thus establishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. Such products may also be released into a flooded reactor cavity during refueling operations or into the coolant circulated through pools where the spent fuel assemblies are stored.
Under these circumstances, it is difficult to detect and remove failed fuel rods because they are part of an integral assembly of guide tubes welded to the top and bottom nozzles. Consequently, to gain access to individual fuel rods, it is necessary to remove the affected assembly from the nuclear reactor core and then break the welds which secure the top and bottom nozzles to the control rod guide thimbles. In so doing, the destructive action often renders the fuel assembly unfit for further use in a reactor because of the damage done to both the guide thimbles and the top and bottom nozzles.
In view of the high costs associated with replacing defective and damaged parts, both domestic and foreign utilities have indicated an interest in reconstitutable fuel assemblies in order to minimize both their operating and maintenance expenses.